A well-known type of marine controllable pitch propeller comprises a pitch-adjusting mechanism in the propeller hub coupled to an inboard hydraulic cylinder by a force rod that extends through the propeller shaft. Usually, the control system for the propeller includes a feedback device that monitors the pitch of the propeller blades, and the inclusion of the feedback device accounts for the conventional use of the term "servo" to refer to the inboard hydraulic cylinder that actuates the pitch-controlling mechanism of the propeller. In a typical inboard servo installation the servo is interposed in the propeller shaft aft of the gear box, although in at least one commercially available system the servo is built into the output gear of the gear box. In either case the servo rotates with the shaft. To provide feedback to the control system, it is conventional to detect the longitudinal position of the force rod, which is indicative of the setting of the pitch-setting mechanism and, therefore, the pitch of the propeller blades. Because the propeller shaft and the force rod are rotating and the force rod moves axially within the rotating shaft, the feedback device commonly includes a coupling between the rotating force rod and a non-rotating feedback output element consisting of a special tubular coupling in the shaft having longitudinal slots of a length at least equal to the working stroke of the force rod, arms projecting from the force rod out through the slots, a rotating ring coupled to the arms and a follower riding in an external track on the ring.
Marine controllable pitch propeller systems also usually make some provision for so-called "emergency lock-up" of the propeller--fixing a predetermined ahead pitch of the blades by a mechanical device--in case of some malfunction in the servo or the control system. One known arrangement makes use of the same output device described above and takes the form of blocks that are inserted in the slots of the shaft coupling for the arms on the force rod. The blocks are generally stowed nearby the output device, but for use they have to be located, removed from stowage and properly placed where they are supposed to go. There are ample opportunities for things to go wrong, such as misplacement or loss of the blocks, difficulty in locating the blocks even when they are where they should be, and erroneous installation. The use of blocks also presupposes the existence of adequate means other than the servo itself, which may be inoperable, to move the propeller blades to the emergency lock-up pitch setting. One such means are jacks positioned between suitable reaction abutments and the servo output ring--the jacks are also subject to loss, misplacement and improper use and constitute still another piece-part in the ship's equipment.